DETERGENT COMPOSITIONS CONTAINING SILVER ANTI-TARNISHING AGENTS

Abstract: A bleaching detergent composition which prevents or inhibits tarnishing of silver comprises: (a) a bleaching agent selected from a peroxygen or peroxygen-yielding compound, a hypohalite or hypohalite-yielding compound, or a salt thereof, or mixtures thereof; (b) an anti-tarnishing agent selected from: (i) a purine class compound; (ii) cyanuric acid or isocyanuric acid or a salt thereof; (iii) a 1,3-N azole compound; (iv) a mixture of any of the above compounds (i), (ii) or (iii); (c) optionally a builder; (d) optionally, a surfactant; wherein the composition exhibits a pH value in the range from 7 to 13, and wherein the anti-tarnishing agent has a pKa value below the pH value of an aqueous solution of the composition.

Description: DETERGENT COMPOSITIONS CONTAINING SILVER ANTI-TARNISHING AGENTS FIELD OF THE INVENTION This invention relates to bleaching detergent compositions which include silver anti-tamishing agents. More particularly, the invention relates to detergent compositions based on peroxygen and/or hypohalite bleaching agents and which include one or more particular organic heterocyclic species as a silver anti-tarnishing agent. BACKGROUND OF THE INVENTION AND PRIOR ART Silver is chemically the most reactive element among the noble metals and tarnishes readily on exposure to sulphur bearing atmospheres. Because of its electronic state, silver exhibits a drastically different chemical behaviour from, for instance, copper, although both metals are in the same group of the Periodic Table. Thus, silver tarnishing is very different from corrosion of other metals. Tarnishing, sometimes referred to as discolouring, is caused by a silver oxidation process in which sulphide is formed. Food such as onions, mustard and eggs which contain organic sulphur compounds are also known to tarnish silver. See Singh et al., "Silver Tarnishing and its Prevention - A Review" Anti-corrosion Methods and Materials, Vol. 30 (July 1983), pp. 4-8. Silver tarnishing is also known to occur when an oxygen bleaching agent used in detergent compositions oxidizes the silver to silver oxide. This oxidation process causes surface-blackening of the silver, leaving undesirable tarnishing of silverware when machine-dishwashed, for example. The use of organic compounds to enhance the resistance of a silver surface to tarnishing has been described in Singh et al, supra pp. 5-6. Certain triazoles, particularly benzotriazole and its variations, are useful as corrosion- inhibiting agents in detergent compositions as described in CH 673033 (1990) and U.S. 4,321,166 (Procter & Gamble, 1982). U.S. Patents Nos. 2,618,606 (Procter & Gamble) and 2,618,608 (Procter & Gamble) disclose the use of derivatives of triazole, including 1,2,3-triazole, imidazole and pyrazole, as discolouration inhibitors used in detergent compositions for non-ferrous metals, such as copper and brass. The detergents described do not, however, contain a bleaching agent which complicates silver tarnish inhibition. The patents further state that a mixture of copper inhibitors are required to prevent tarnishing over a broad pH range. Conventional detergents, particularly automatic machine dishwashing detergents, are generally formulated with chlorine bleaching agents in a high alkaline pH range. During washing, certain such chlorine bleaches (e.g. chloroisocyanurate) react to form isocyanuric acid and thus do not greatly effect silver discolouration. Detergent compositions are, however, increasingly being based on peroxygen bleaching agents and are being formulated to be milder to produce more environmentally- friendly products. The problem of tarnishing of silver and silver-plated articles has thus become more severe. It is thus a primary object of the present invention to provide a peroxygen and/or hypohalite bleach-based detergent composition containing one or more compounds which prevent or ameliorate the problem of silver tarnishing, particularly in the context of a machine dishwashing environment, especially in an alkaline washing medium. It is a further object of the invention to provide an environmentally-friendly detergent composition which prevents or inhibits tarnishing of silver or silver-plated articles. Another object of the invention is to provide a method for washing silver or silver-plated articles without discolouring them. A further object of the invention is to protect a silver or silver-plated article against tarnishing from organic foodstuffs with which it comes into contact. Silver anti-tarnishing bleaching detergent compositions formulated for use in automatic dishwashing machines or fabric washing machines which are stable in a variety of physical forms, including liquid, powder, flakes, etc. , is another object of the invention. SUMMARY OF THE INVENTION Accordingly, in a first aspect the present invention provides a bleaching detergent composition comprising: (a) from about 1 to about 20% by weight of a bleaching agent selected from a peroxygen or peroxygen-yielding compound, a hypohalite or hypohalite-yielding compound, or a salt thereof, or mixtures thereof; (b) from about 0.05 to about 10% by weight of an anti- tarnishing agent selected from: (i) a purine class compound of the following formula (I) : wherein X 1 is nitrogen or C-R 3 , Y 1 is nitrogen or C-R 4 , and R 1 , R 2 , R 3 and R 4 are each independently hydrogen, hydroxy, alkoxy, amine, straight or branched chain alkyl having 1 to 20 carbon atoms, amido, amidoalkyl, alkylthio, alkenyl or hydroxyalkyl; (ii) cyanuric acid or isocyanuric acid or a salt thereof; (iii) a 1,3-N azole compound of the following formula (II) ; wherein X 2 is C-R 7 or nitrogen provided Y 2 is also nitrogen, Y 2 is nitrogen or C-R 6 , and R s , R 6 and R 7 are each independently hydrogen, amine, amido, straight or branched chain alkyl having from 1 to 20 carbon atoms, an amino- or carboxylic-containing chain, alkoxy, alkylthio, hydroxy, hydroxyalkyl, alkenyl, or R' and R 6 taken together form an unsubstituted or substituted aryl group; or a salt thereof; (iv) a mixture of any of the above compounds (i) , (ii) or (iϋ) ; (c) optionally from about 1 to about 75% by weight of a detergency builder; and (d) optionally, from about 0.01 to about 40% by weight of a surfactant; wherein the composition exhibits a pH value in the range from about 7 to about 13, especially from about 7 to about 11, and wherein the anti-tarnishing agent has a pK a value below the pH value of an aqueous solution of the composition. In a second aspect, the invention provides a method of washing a silver or silver-plated article whilst preventing or inhibiting tarnishing thereof during or as a result of the washing process, the method comprising washing the said article with a bleaching detergent composition according to the first aspect of the invention defined above. In a third aspect, the invention provides the use of a compound selected from any of (i) to (iv) defined above in accordance with the first aspect of the invention, as a silver anti-tarnishing agent in a peroxygen- and/or hypohalite based bleaching detergent composition having a pH value in the range from about 7 to about 13. The various aspects of the invention, and preferred embodiments thereof, will now be described in detail. DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS Anti-tarnishing agent The silver anti-tarnishing agent of compositions of the invention is a compound or a mixture of two or more compounds selected from any of the following: (i) a purine class compound of the following formula (I): wherein X 1 is nitrogen or C-R 3 , Y 1 is nitrogen or C-R 4 , and R 1 , R 2 , R 3 and R 4 are each independently hydrogen, hydroxy, alkoxy, amine, straight or branched chain alkyl having 1 to 20 carbon atoms, amido, amidoalkyl, alkylthio, alkenyl or hydroxyalkyl; (ii) cyanuric acid or isocyanuric acid or a salt thereof; (iii) a 1,3-N azole compound of the following formula (II) ; wherein X 2 is C-R 7 or nitrogen provided Y 2 is also nitrogen, Y 2 is nitrogen or C-R 6 , and R 5 , R 6 and R 7 are each independently hydrogen, amine, amido, straight or branched chain alkyl having from 1 to 20 carbon atoms, an amino- or carboxylic-containing chain, alkoxy, alkylthio, hydroxy, hydroxyalkyl, alkenyl, or R s and R 6 taken together form an unsubstituted or substituted aryl group; or a salt thereof; with the proviso that the compound(s) have a pK a value which is below, preferably at least 1 unit below (especially in the case of the purine class compounds) , more preferably at least 2 units below, a pH value of an aqueous solution (preferably a 1% aqueous solution) of the composition in which the compound(s) is/are incorporated. Most preferably, the pK a value of the anti-tarnishing compound(s) is at least 2 and up to 6 units below the pH value of the aqueous solution of the composition. As used herein, the term "pK a " means a pH value at which 50% of the heterocyclic ring moieties of the compound (particularly the five- embered rings in the case of the purine class compounds) are in anionic form. As used herein, the term "purine class compound" includes not only compounds of formula (I) above, but also tautomeric forms of these compounds. Preferred compounds of formula (I) include those wherein X 1 is nitrogen and Y 1 is C-R 4 and R 1 , R 2 , R 3 and R 4 are each independently hydrogen, hydroxy, alkoxy, oxygen, alkylthio, amine, amido or lower alkyl having from 1 to 6 carbon atoms. Especially preferred compounds of formula (I) include purine, adenine, guanine, 6-mercaptopurine, xanthine, hypoxanthine, uric acid, and allopurinol. The above purine class compounds are available commercially, for example from Aldrich Chemical Co. of Milwaukee, Wisconsin, USA. As used herein, the term "cyanuric acid" means 1,3,5- triazine-2,4 , 6 (IH, 3H, 5H) -trione, normal cyanuric acid, sym- triazinetriol, 2 , 4, 6-trihydroxy-l, 3 ,5-triazine, tricyanic acid or trihydroxycyanidine. Corresponding possibilities apply to the term "isocyanuric acid" as used herein, and within the scope of both terms are salts thereof. Preferably, the cyanuric acid or isocyanuric acid, or a salt thereof, as an anti-tarnishing agent in the present invention, is an unsubstituted such species. Cyanuric acid is available commercially, for example also from Aldrich Chemical Co. of Milwaukee, Wisconsin, USA. Preferred 1,3-N azole compounds of formula (II) above include those wherein X 2 is C-R 7 , X 2 is nitrogen provided Y is nitrogen, Y 2 is C-R 6 and R 5 and R 6 taken together form an aryl or a substituted aryl group. Other preferred compounds of formula (II) include those wherein Y 2 is C-R 6 and R 5 and R 6 are each independently hydrogen, amine, amido, straight or branched chain alkyl having from 1 to 6 carbon atoms, alkoxy, alkylthio, hydroxy, alkenyl or an amino- or carboxylic-containing moiety. Especially useful compounds of formula (II) include imidazole, benzimidazole, tetrazole, 5-aminotetrazole, 1, 2 ,4-triazole, 3-amino-l,2,4-triazole and histidine. 1,3-N azole compounds of formula (II) above useful in the invention are commercially available, for example also from Aldrich Chemical Co. of Milwaukee, Wisconsin, USA. In the above formulae (I) and (II) , a group defined as being "alkyl" preferably means a group having from 1 to 20 carbon atoms, more preferably from 1 to about 6 carbon atoms, most preferably from 1 to about 4 carbon atoms. In formulae (I) and (II) above, those defined groups which are substituted alkyl groups preferably have an alkyl chain length of from 1 to about 5 carbon atoms, more preferably from 1 to about 3 carbon atoms. The term "aryl" includes aryl groups containing one or more heteroatoms such as S, N or O, preferably N. The term "substituted aryl" includes an aryl group substituted with one or more of the following groups: straight or branched chain alkyl having from 1 to 20 carbon atoms, hydroxy, alkoxy or alkenyl. In the making of the present invention, it was surprising to discover that the above defined compounds (i) to (iii) provide beneficial anti-tarnishing properties to silver and silver-plated articles in the context of the subject active oxygen- or active halogen-based bleaching detergent compositions. In contrast, compositions containing certain 1-N azoles (e.g. pyrrole), 1,2-N azoles (e.g. indazole, pyrazole) and pyrimidine have been observed to be substantially ineffective in preventing heavy tarnishing of similar silver and silver-plated articles. This observation was particularly surprising in view of the fact that most of the 1-N and 1,2-N azole compounds did prevent copper tarnishing under similar conditions. Without being limited by theory, it is proposed that the compounds used in the present invention form a complex with the silver molecules at the surface of the article to form a protective film which prevents or diminishes its tendency to tarnish in an oxidative environment. In the anti-tarnishing bleaching detergent compositions of the invention, the one or more compounds forming the anti- tarnishing agent are present in a (total) amount of from about 0.05 to about 10% by weight, preferably from about 0.25 to about 2.5% by weight, most preferably from about 0.75 to about 2% by weight. EH The pH of an aqueous solution (preferably a 1% solution) of the compositions in accordance with the invention should be from about 7 to about 13, more preferably from about 7 to about 11, most preferably from about 7 or 8 to about 10. Peroxy bleaching agent Peroxygen or peroxygen-yielding bleaching agents for use in the compositions of the invention include organic peroxy acids and diacylperoxides. Typical monoperoxy acids useful herein include alkyl peroxy acids and aryl peroxy acids such as: (i) peroxybenzoic acid and ring-substituted peroxybenzoic acids, e.g., peroxy-alpha-naphthoic acid, and magnesium monoperphthalate; (ii) aliphatic and substituted aliphatic monoperoxy acids, e.g., peroxylauric acid, peroxystearic acid, epsilon-phthalimido peroxyhexanoic acid, o-carboxybenzamido peroxyhexanoic acid, N-nonenylamidoperadipic acid and N- nonenylamidopersuccinic acid. Typical diperoxy acids useful herein include alkyl diperoxy acids and aryldiperoxy acids, such as: (iii) 1, 12-diperoxydodecanedioic acid (iv) 1, 9-diperoxyazelaic acid (v) diperoxybrassylic acid; diperoxysebacic acid and diperoxyisophthalic acid (vi) 2-decyldiperoxybutane-l,4-dioic acid (vii) N,N-terephthaloyl-di(6-aminopercaproic acid) . A typical diacylperoxide useful herein includes dibenzoylperoxide. Inorganic peroxygen compounds are also suitable for use in the present invention. Examples of such useful materials include salts of monopersulphate, perborate monohydrate, perborate tetrahydrate, and percarbonate. Preferred organic oxygen bleaching agents include epsilon- phthalimidoperoxyhexanoic acid, o-carboxybenzamidoperoxy hexanoic acid, and mixtures thereof. When an oxygen bleaching agent is used as the bleaching agent in compositions of the invention, it is present therein in an amount from about 1 to about 20% by weight, preferably from about 1 to about 15% by weight, most preferably from about 2 to about 10% by weight. The oxygen bleaching agent may be incorporated directly into the formulation or may be encapsulated by any suitable encapsulation technique known in the art to produce stable capsules in alkaline liquid formulations. A preferred encapsulation method is described in U.S. Patent No. 5,200,236 (Lang et al) , the disclosure of which is incorporated herein by reference. In the patented method, the bleaching agent is encapsulated as a core in a paraffin wax material having a melting point from about 40°C to about 50°C. The wax coating has a thickness of from 100 to 1500 microns. Oxygen bleaching agent systems which may be employed in the present invention may if desired or if necessary employ one or more peroxyacid bleach precursors. Suitable peroxygen peracid precursors for peroxy bleach compounds have been amply described in the literature, including United Kingdom Patents Nos. 836,988; 855,735; 907,356; 907,358; 907,950; 1,003,310 and 1,246,339; and U.S. Patents Nos. 3,332,882 and 4,128,494. Typical examples of precursors are polyacylated alkylene diamines, such as N,N,N' ,N'-tetraacetylethylene diamine (TAED) and N,N,N' ,N'-tetraacetylmethylene diamine (TAMD) ; acylated glycolurils, such as tetraacetylglycoluril (TAGU) ; triacetylcyanurate, sodium sulphophyl ethyl carbonic acid ester, sodium acetyloxybenzene sulphonate (SABS) , sodium nonanoyloxybenzene sulphonate (SNOBS) and choline sulphophenyl carbonate. Peroxybenzoic acid precursors are known in the art, e.g., as described in GB-A-836,988. Examples of suitable precursors are phenylbenzoate; phenyl p-nitrobenzoate; o-nitrophenyl benzoate; o-carboxyphenyl benzoate; p-bromo-phenylbenzoate; sodium or potassium benzoyloxy benzenesulphonate; and benzoic anhydride. Preferred peroxygen bleach precursors are sodium p- benzoyloxybenzene sulphonate, N,N,N' ,N'-tetraacetylethylene diamine, sodium nonanoyloxybenzene sulphonate and choline sulphophenyl carbonate. Halogen bleaching agent Hypohalite or hypohalite-yielding bleaching agents for use in the compositions of the invention include hypohalite salts per se or compounds which yield hypohalite anions in aqueous alkaline conditions. Such materials are preferably incorporated into compositions of the invention in the form of dry, particulate, water-soluble anhydrous inorganic salts. Examples of suitable hypohalite salts for use in the invention include lithium, sodium or calcium hypochlorite and hypobromite, and halogenated (e.g. with chlorine or bromine) trisodium phosphate. Sodium hypochlorite is particularly preferred for liquid compositions in accordance with the invention. Hypohalite-yielding compounds suitable for use in the invention include, for example, chloramines, chloramides, chlorimines, chlorosulphonamides, and chlorohydantoinε (though preferably not N-chloro imides which are harsher and less environmentally friendly than oxygen bleaching agents and other hypohalite bleaching agents such as those above) , and active halogen compounds corresponding to any of these but which contain bromine instead of chlorine, or contain a mixture of chlorine and bromine. Also suitable for use in the invention as a bleaching agent are halogenated isocyanuric acids, such as trichloroisocyanuric acid, dichloroisocyanuric acid, and salts thereof, especially the sodium salts. Commercial sources of chlorinated isocyanuric acids include, for example, ACL-59 (trade mark) supplied by Mansanto Company, and Clearon CDB-56 (trade mark) supplied by Olin Corporation. When a halogen-based bleaching agent such as those described above is used as the bleaching agent in compositions of the invention, it is present therein in a similar amount to that when an oxygen-based bleaching agent is used, namely in an amount of from about 1 to about 20% by weight, preferably from about 1 to about 15% by weight, most preferably from about 2 to about 10% by weight. In embodiments where a mixture of oxygen and halogen bleaching agents are optionally employed, each component is used in an appropriate amount, such that the total amount of bleaching agent present also falls within the essential and preferred narrower ranges defined above with respect to each type of bleaching agent separately. Detergent Builder Materials The compositions of this invention may contain all manner of detergent builders commonly taught for use in automatic dishwashing or other cleaning compositions. The builder component can include any of the conventional inorganic and organic water-soluble builder salts or mixtures thereof and, when present, is included in the composition preferably in an amount of from about 1 to about 75% by weight, preferably from about 5 to about 70% by weight. Typical examples of phosphorus-containing inorganic builders include water-soluble (especially alkali metal) pyrophosphates, orthophosphates and polyphosphates. Specific examples of inorganic phosphate builders include sodium and potassium tripolyphosphates, phosphates, pyrophosphates and hexametaphosphates. Suitable examples of non-phosphorus-containing inorganic builders include water-soluble (especially alkali metal) carbonates, bicarbonates, sesquicarbonates, borates, silicates, metasilicates, and crystalline and amorphous aluminosilicates. Specific examples include sodium carbonates (with or without calcite seeds) , potassium carbonate, sodium and potassium bicarbonates, silicates and zeolites. Particularly preferred inorganic builders may be selected from the group consisting of sodium tripolyphosphate, potassium pyrophosphate, sodium carbonate, potassium carbonate, sodium bicarbonate, sodium silicate and mixtures thereof. When present in the compositions of the invention, sodium tripolyphosphate builder concentrations may range from about 2 to about 40% by weight, preferably from about 5 to about 30% by weight of the composition. Sodium carbonate and bicarbonate, when present, may range from about 5 to about 50% by weight, preferably from about 10 to about 30% by weight of the composition. Sodium tripolyphosphate and potassium pyrophosphate are preferred builders in gel formulations, where they may be used in an amount of from about 3 to about 30% by weight, preferably from about 10 to about 20% by weight. Organic detergent builders may also be used in the compositions of the invention. Examples of organic builders include alkali metal citrates, succinates, malonates, fatty acid sulphates, fatty acid carboxylates, nitrilotriacetates, phytates, phosphonates, alkanehydroxyphosphonates, oxydisuccinates, alkyl and alkenyl disuccinates, oxydiacetates, carboxymethyloxy succinates, ethylenediamine tetraacetates, tartrate monosuccinates, tartrate disuccinates, tartrate monoacetates, tartrate diacetates, oxidized starches, oxidized heteropolymeric polysaccharides, polyhydroxysulphonates, polycarboxylates such as polyacrylates, polymaleates, polyacetates, polyhydroxyacrylates, polyacrylate/polymaleate and polyacrylate/polymethacrylate copolymers, aminopolycarboxylates and polyacetal carboxylates such as those described in U.S. Patents Nos. 4,144,226 and 4,146,495. Alkali metal citrates, oxydisuccinates, polyphosphonates and acrylate/maleate copolymers are especially preferred organic builders. When present, they are preferably included in an amount of from about 1 to about 35% by weight of the composition. The foregoing detergent builders are intended to illustrate by way of example only, and not to limit, the types of builders which may be employed in the compositions of the present invention. Surfactants The compositions of this invention may contain one or more surfactant materials. Useful surfactants include anionic, nonionic, cationic, amphoteric and zwitterionic types, and mixtures thereof. Such surfactants are well known in the detergent art and are described at length in "Surface Active Agents and Detergents", Vol. II, by Schwartz, Perry & Birch, Interscience Publishers, Inc. 1959, incorporated herein by reference. Anionic synthetic detergents can be broadly described as surface-active compounds with one or more negatively charged functional groups. Soaps are included within this category. A soap is a C 8 -C 22 alkyl fatty acid salt of an alkali metal, alkaline earth metal, ammonium, alkyl- substituted ammonium or alkanolammonium salt. Sodium salts of tallow and coconut fatty acids and mixtures thereof are most common. Another important class of anionic compounds are the water-soluble salts, particularly the alkali metal salts, of organic sulphur reaction products having in their molecular structure an alkyl radical containing from about 8 to 22 carbon atoms and a radical selected from the group consisting of sulphonic and sulphuric acid ester radicals. Organic sulphur-based anionic surfactants include the salts of C 10 -C, 6 alkylbenzene sulphonates, C I0 -C 22 alkane sulphonates, C H) -C 22 alkyl ether sulphates, C 10 -C 22 alkyl sulphates, C 4 -C 10 dialkylsulphosuccinates, C 10 -C 22 acyl isethionates, alkyl diphenyloxide sulphonates, alkyl napthalene sulphonates, and 2-acetamido hexadecane sulphonates. Organic phosphate-based anionic surfactants include organic phosphate esters such as complex mono- or diester phosphates of hydroxyl- terminated alkoxide condensates, or salts thereof. Included in the organic phosphate esters are phosphate ester derivatives of polyoxyalkylated alkylaryl phosphate esters of ethoxylated linear alcohols and ethoxylates of phenol. Also included are nonionic alkoxylates having a sodium alkylenecarboxylate moiety linked to a terminal hydroxyl group of the nonionic through an ether bond. Counterions to the salts of all the foregoing may be those of alkali metal (especially sodium) , alkaline earth metal, ammonium, alkanolammonium and alkylammonium types. Nonionic surfactants can be broadly defined as surface- active compounds with one or more uncharged hydrophilic substituents. A major class of nonionic surfactants are those compounds produced by the condensation of alkylene oxide groups with an organic hydrophobic material which may be aliphatic or alkyl aromatic in nature. The length of the hydrophilic or polyoxyalkylene radical which is condensed with any particular hydrophobic group can be readily adjusted to yield a water-soluble compound having the desired degree of balance between hydrophilic and hydrophobic elements. Illustrative, but not limiting, examples of various suitable nonionic surfactant types are: (a) polyoxyethylene or polyoxypropylene condensates of aliphatic carboxylic acids, whether linear- or branched- chain and unsaturated or saturated, containing from about 8 to about 18 carbon atoms in the aliphatic chain and incorporating from about 2 to about 50 ethylene oxide and/or propylene oxide units. Suitable carboxylic acids include "coconut" fatty acids (derived from coconut oil) which contain an average of about 12 carbon atoms, "tallow" fatty acids (derived from tallow-class fats) which contain an average of about 18 carbon atoms, palmitic acid, myristic acid, stearic acid and lauric acid; (b) polyoxyethylene or polyoxypropylene condensates of aliphatic alcohols, whether linear- or branched-chain and unsaturated or saturated, containing from about 6 to about 24 carbon atoms and incorporating from about 2 to about 50 ethylene oxide and/or propylene oxide units. Suitable alcohols include coconut fatty alcohol, tallow fatty alcohol, lauryl alcohol, myristyl alcohol and oleyl alcohol. Particularly preferred nonionic surfactant compounds in this category are the "Neodol"-type products, a registered trademark of the Shell Chemical Company. Also included within this category are nonionic surfactants having the following formula (III) : R- (CH 2 CHO) x (CH 2 CH 2 0) y (CH 2 CHO) z H (III) R 1 R 2 wherein R is a linear alkyl hydrocarbon radical having an average of 6 to 18 carbon atoms, R 1 and R 2 are each linear alkyl hydrocarbons of about 1 to about 4 carbon atoms, x is a integer of from 1 to 6, y is an integer of from 4 to 20 and z is an integer from 4 to 25. One preferred nonionic surfactant of formula I is Poly- Tergent SLF-18 (trade mark) , from the Olin Corporation, New Haven, Connecticut, USA, having a composition of the above formula where R is a C 6 -C K) linear alkyl mixture, R 1 and R 2 are methyl, x averages 3, y averages 12 and z averages 16. Also suitable are alkylated nonionics as are described in U.S. Patent No. 4,877,544 (Gabriel et al.), incorporated herein by reference. Another family of nonionic surfactants included within this category are compounds of the following formula (IV) : R-(CH 2 CH 2 0) a H (IV) wherein R 3 is a C 6 -C 24 linear or branched alkyl hydrocarbon radical and q is a number from 2 to 50; more preferably R 3 is a C 3 -C 18 linear alkyl mixture and q is a number from 2 to 15; (c) polyoxyethylene or polyoxypropylene condensates of alkyl phenols, whether linear- or branched- chain and unsaturated or saturated, containing from about 6 to 12 carbon atoms and incorporating from about 2 to about 25 moles of ethylene oxide and/or propylene oxide; (d) polyoxyethylene derivatives of sorbitan mono-, di-, and tri-fatty acid esters wherein the fatty acid component has from 12 to 24 carbon atoms. The preferred polyoxyethylene derivatives are of sorbitan monolaurate, sorbitan trilaurate, sorbitan monopalmitate, sorbitan tripalmitate, sorbitan monostearate, sorbitan monoisostearate, sorbitan tripalmitate, sorbitan monostearate, sorbitan monoisostearate, sorbitan tristearate, sorbitan monooleate, and sorbitan trioleate. The polyoxyethylene chains may contain between about 4 and 30 ethylene oxide units, preferably about 20. The sorbitan ester derivatives contain 1, 2 or 3 polyoxyethylene chains dependent upon whether they are mono-, di- or tri-acid esters; (e) polyoxyethylene-polyoxypropylene block copolymers having the following formula (V) : HO(CH 2 CH 2 0) a (CH(CH 3 )CH 2 0) b (CH 2 CH 2 0) c H (V) or the following formula (VI) : HO (CH ( CH 3 ) CH 2 0) d ( CH 2 CH,0) e ( CHCH 3 CH 2 0) f H (VI ) wherein a, b, c, d, e and f are integers from 1 to 350, reflecting the respective polyethylene oxide and polypropylene oxide blocks of said polymer. The polyoxyethylene component of the block polymer constitutes at least about 10% of the block polymer. The material preferably has a molecular weight of between about 1,000 and 15,000, more preferably from about 1,500 to about 6,000. These materials are well known in the art. They are commercially available for example under the trademarks "Pluronic" and "Pluronic R" from BASF corporation; (f) Alkyl glycosides having the following formula (VII) : R 4 0(R s 0) n (Z') p (VII) wherein R 4 is a monovalent organic radical (e.g., a monovalent saturated aliphatic, unsaturated aliphatic or aromatic radical such as alkyl, hydroxyalkyl, alkenyl, hydroxyalkenyl, aryl, alkylaryl, hydroxyalkylaryl, arylalkyl, alkenylaryl, arylalkenyl, etc.) containing from about 6 to about 30 (preferably from about 8 to 18, more preferably from about 9 to about 13) carbon atoms; R 5 is a divalent hydrocarbon radical containing from 2 to about 4 carbon atoms such as ethylene, propylene or butylene (most preferably the unit (R 5 0) n represents repeating units of ethylene oxide, propylene oxide and/or random or block combinations thereof) ; n is a number having an average value of from 0 to about 12; Z 1 represents a moiety derived from a reducing saccharide containing 5 or 6 carbon atoms (most preferably a glucose unit) ; and p is a number having an average value of from 0.5 to about 10, preferably from about 0.5 to about 5. Examples of commercially available materials from Henkel Kom anditgesellschaft Aktien of Dusseldorf, Germany include: APG (trade mark) 300, 325 and 350, with R 4 being C 9 -C π , n is 0 and p is 1.3, 1.6 and 1.8-2.2, respectively; APG 500 and 550 with R 4 is C,,-C 13 , n is 0 and p is 1.3 and 1.8-2.2, respectively; and APG 600 with R 4 being C 12 -C l4 , n is 0 and p is 1.3. Particularly preferred is APG 600; (g) Amine oxides having the following formula (VI II ) : R 5 R 6 R 7 N = O (VIII ) wherein R 5 , R 6 and R 7 are saturated aliphatic radicals or substituted saturated aliphatic radicals. Preferable amine oxides are those wherein R 5 is an alkyl chain of about 10 to about 20 carbon atoms and R° and R 7 are methyl or ethyl groups or both R 5 and R 6 are alkyl chains of about 6 to about 14 carbon atoms and R 7 is a methyl or ethyl group. Amphoteric synthetic detergents can be broadly described as derivatives of aliphatic and tertiary amines, in which the aliphatic radical may be straight or branched chain and wherein one of the aliphatic substituents contain from about 8 to about 18 carbons and one contains an anionic water-solubilizing group, i.e., carboxy, sulpho, sulphato, phosphato or phosphono. Examples of compounds falling within this definition are sodium 3-dodecylamino propionate and sodium 2-dodecylamino propane sulphonate. Zwitterionic synthetic detergents can be broadly described as derivatives of aliphatic quaternary ammonium, phosphonium and sulphonium compounds in which the aliphatic radical may be straight or branched chain, and wherein one of the aliphatic substituents contains from about 8 to about 18 carbon atoms and one contains an anionic water- solubilizing group, e.g., carboxy, sulpho, sulphato, phosphato or phosphono. These compounds are frequently referred to as betaines. Besides alkyl betaines, alkyl amino and alkyl amido betaines are encompassed herewithin. The one or more surface active materials forming the surfactant component of compositions of the invention, when present, constitute from about 0.01 to about 40% by weight of the composition. The amount of surfactant, if present, will generally be within this range, but the actual amount used may depend upon the type of surfactan (s) employed. For instance, anionic and/or nonionic surfactants making up the total surfactant concentration may typically be present in an amount of from about 1 to about 40% by weight of the composition, more preferably from about 2 to about 35% by weight, even more preferably from about 5 to about 30% by weight. Alkyl polyglycosides as the surfactant component, however, may typically be present in lower concentrations, such as in an amount from about 0.01 to about 20% by weight, preferably from about 0.5 to about 10% by weight, optimally between about 1 and about 5% by weight of the composition. OPTIONAL ADDITIONAL INGREDIENTS Silicates The compositions of this invention may optionally contain sodium or potassium silicate in an amount of from about 1 to about 40%, preferably from about 1 to about 20% by weight of the composition. When present, this material is employed as a cleaning ingredient, source of alkalinity, metal corrosion inhibitor and protector of glaze on china tableware. Especially effective is sodium silicate having a ratio of Si0 2 :Na 2 0 of from about 1.0 to about 3.3, preferably from about 2 to about 3.2. Some of the silicate may be in solid form. Filler An inert particulate filler material which is water-soluble may optionally also be present in the compositions of the invention which are in powder form. This material should not precipitate calcium or magnesium ions at the filler use level. Suitable for this purpose are organic or inorganic compounds. Organic fillers include sucrose esters and urea. Representative inorganic fillers include sodium sulphate, sodium chloride and potassium chloride. A preferred filler is sodium sulphate. Its concentration may range from 0 to about 60%, preferably from about 10 to about 30% by weight of the composition. Thickeners and Stabilizers Thickeners are often desirable for inclusion in liquid cleaning compositions of the invention. Thixotropic thickeners such as smectite clays including montmorillonite (bentonite) , hectorite, saponite, and the like may be used to impart increased viscosity to such liquid bleaching detergent compositions. Silica, silica gel, and aluminosilicate may also be used as thickeners. Salts of polyacrylic acid (of molecular weight of from about 300,000 up to 6 million and higher) , including polymers which are cross-linked, may also be used either alone or in combination with other thickeners. Use of clay thickeners for automatic dishwashing compositions is disclosed, for example, in U.S. Patents Nos. 4,431,559; 4,511,487; 4,740,327; and 4,752,409. Commercially available synthetic smectite clays include Laponite (trade mark) supplied by Laporte Industries. Commercially available bentonite clays include Korthix H and VWH ex Combustion Engineering, Inc.; Polargel T ex American Colloid Co.; and Gelwhite clays (particularly Gelwhite GP and H) ex English China Clay Co. Polargel T is preferred as imparting a more intense white appearance to the composition than other clays. The amount of clay thickener employed in the compositions, when it is present, is preferably from about 0.1 to about 10% by weight, preferably from about 0.5 to about 5% by weight. Use of salts of polymeric carboxylic acids is disclosed, for example, in GB-A-2, 164, 350, and in U.S. Patents Nos. 4,859,358 and 4,836,948. For liquid formulations with a gel appearance and rheology, particularly if a clear gel is desired, a chlorine-stable polymeric thickener is particularly useful. U.S. Patent No. 4,260,528 discloses natural gums and resins for use in clear autodishwashing detergent compositions, which are not chlorine-stable. Cross-linked acrylic acid polymers manufactured by, for example, B.F.Goodrich and sold under the tradename "Carbopol" have been found to be effective for producing clear gels, and Carbopol 940 and 617, having a molecular weight of about 4,000,000, are particularly preferred for maintaining high viscosity with excellent chlorine stability over extended periods. Further suitable chlorine-stable polymeric thickeners are described in U.S. Patent No. 4,867,896, incorporated herein by reference. When it is present, the amount of thickener employed in the compositions of the invention is preferably from 0 to about 5% by weight, preferably from about 0.5 to about 3% by weight. Stabilizers and/or co-structurants such as long-chain calcium and sodium soaps and C 12 to C 18 ■sulphates which may optionally be incorporated in compositions of the invention are detailed in U.S. Patents Nos. 3,956,158 and 4,271,030 and the use of other metal salts of long-chain soaps is detailed in U.S. Patent No. 4,752,409. Other co- structurants include Laponite (trade mark) and metal oxides and their salts, as described in U.S. 4,933,101, incorporated herein by reference. The amount of stabilizer which may be used in liquid compositions of the invention is preferably from about 0.01 to about 5% by weight of the composition, preferably from about 0.01 to about 2% by weight. Such stabilizers are optional in gel formulations. Co-structurants which are found especially suitable for gels include compounds containing trivalent metal ions which may be present in an amount of from about 0.01 to about 4% by weight of the composition, and Laponite and/or water-soluble structuring chelants, which may be pre_ nt in an amount of from about 1 to about 60% by weight. The =e c structurants are more fully described in EP-A-032320S the disclosure of which is incorporated herein by reference. Defoamer Formulations of the cleaning composition of the invertion comprising surfactant may further include a defoamer. Suitable defoa ers include mono- and distearyl acid phosphate, silicone oil and mineral oil. Even if the composition includes only defoaming surfactant, the defoamer may still assist in the minimizing of foam wnich food soils can generate. If a defoamer is present, the compositions may include from about 0.02 to about 2% by weight thereof, more preferably from about 0.05 to about 1.0% by weight thereof. Other adjunct ingredients Minor amounts of various other optional components may be present in the cleaning compositions of the invention. These include: bleach scavengers including (but not limited to) sodium bisulphite, sodium perborate, reducing sugars, and short-chain alcohols; solvents and hydrotropes such as ethanol, isopropanol and xylene sulphonates; flow control agents (in granular forms of the composition) ; enzyme- stabilizing agents; soil-suspending agents; anti- redeposition agents; anti-tarnish agents; anti-corrosion agents; colourants; other functional additives; perfumes. The pH of cleaning compositions of the invention may be adjusted as desired or as necessary by addition of strong acid or base. Such alkalinity or buffering agents include, for example, sodium carbonate and sodium borate. Enzymes Enzymes capable of facilitating the removal of soils from a substrate may also optionally be present in compositions of the invention, preferably in an amount of from 0 to about 10% by weight, preferably from about 1 to about 5% by weight. Such enzymes include proteases (e.g., Alcalase (trade mark) , Savinase (trade mark) and Esperase (trade mark) from Novo Industries A/S) , amylases (e.g. , Termamyl (trade mark) from Novo Industries A/S), lipases (e.g., Lipolase (trade mark) from Novo Industries A/S) , oxidases, and mixtures thereof. The following examples more fully illustrate the invention in its various aspects and preferred embodiments thereof. All parts, percentages and proportions referred to herein and in the appended claims are by weight unless otherwise indicated. EXAMPLE 1 The following seven machine dishwashing compositions were prepared as follows: Table 1 1 An acrylic acid/maleic acid copolymer supplied by BASF Corporation of Parsippany, New Jersey, USA 2 The following different levels of sodium perborate (H 2 0) and TAED (N,N,N ,N / -tetraacetylethylene diamine bleach activator) were used in Example 1: No anti-tarn sh agent was used n Example 1. Tarnish-monitoring experiments to determine silver tarnishing were conducted with compositions a, b, c, d, e, f, and g at a product dosage of 40 grams per run in a European dishwasher, Bauknecht GSF 3162, with an intake of of 5 litres deionized water. The wash program consisted of a pre-wash at 40°C, a mainwash at 55°C, two intermediate rinses, and a final rinse at 65°C. The mainwash pH with these compositions was typically about 8.7. Silver-plated spoons (2 per run) were the monitors used in the test. The spoons were supplied by Oneida Silversmiths, USA. The monitors were washed in a commercially available hand dishwash liquid and rinsed with deionized water and acetone before use. In the dishwasher the monitors were kept apart from each other in the cutlery basket. At the end of a single-run machine program, the monitors were visually analyzed for the presence of colours and the loss of gloss. The spoons washed with composition a were unchanged after the dishwashing process, except for a few stain spots. The stain spots were caused by deposition of non-volatile materials during the evaporation of undrained wash solution in the drying step of the machine program and were not related to a tarnishing (i.e. silver oxidation) process of the monitors. However, the spoons washed with compositions b, c, d, e, f, and g were increasingly tarnished as the levels of bleaching agent were increased. The spoons washed with composition g were so heavily tarnished that they lost their gloss and were turned brown/black. The set of spoons washed with the compositions a, b, c, d, e, f, and g were ranked from zero to six, respectively. This ranking of spoons was used as a reference scale for all subsequent Examples. EXAMPLE 2 Machine dishwashing compositions were prepared as described in Example 1, except that the selected bleaching agent was epsilon-phthalimido peroxyhexanoic acid incorporated at 4% by weight. The compound was supplied by Hoechst AG of Germany. The compositions also contained various anti- tarnish agents incorporated in an amount of 1% by weight, as follows: Table 2 The anti-tarnishing agents were supplied by Aldrich Chemical Co. Tarnish-monitoring experiments as described in Example 1 were conducted with Samples A to H, using two silver-plated spoons as monitors in each of the experiments. The main wash pH in this series of experiments was between 8.8 and 8.5. Anti-tarnish scores ranging from 0 to 6 according to the level of tarnishing were obtained for samples A to H as follows: TABLE 3 It was observed that spoons washed with compositions containing anti-tarnish agents (Samples B to H) either remained unchanged or developed a slightly yellowish appearance during the dishwashing process. In contrast, the control sample A, which did not contain an anti-tarnish agent, produced heavy tarnishing. EXAMPLE 3 A detergent base was prepared having the following formula: Table 4 1 An acrylic acid/maleic acid copolymer supplied by BASF, Germany. 2 N,N,N' ,N'-tetraacetylethylene diamine. 3 A smectite clay supplied by Laporte Industries of Cheshire, England. Various amounts of an adenine compound were dosed into 27 gram samples of the foregoing base as follows: Table 5 As described in Example 1, tarnish monitoring tests were conducted with samples 1 to 5 using two silver-plated spoons as monitors. However, 1 gram/liter of sodium chloride was added to the deionized water used in the experiments. Additionally, 5 grams of egg yolk were dosed in the dishwasher immediately after water intake at the start of the main wash of each experiment. The main wash pH values were typically about 9.8. Silver-tarnishing scores on a scale of 0 to 6 were observed for samples 1 to 5 as follows: TABLE 6 The control sample containing no anti-tarnishing agent gave rise to moderately heavy tarnishing. In contrast, samples 2 to 5 exhibited little to no tarnishing. Optimal anti- tarnishing performance was observed with sample 4 containing 0.25 wt.% adenine. EXAMPLE 4 Six machine dishwashing compositions were prepared as described in Example 1, except that the type of bleaching agent and the amount of purine as the anti-tarnish agent were varied as follows: Table 7 1 A 32% peracetic acid solution, supplied by Aldrich Corporation of Milwaukee, USA was used. 2 Supplied by Hoechst AG, Germany. 3 An 8.2% active Cl 2 solution was used, supplied by Jones Chemicals of Caledonia, NY, USA. Tarnishing-monitoring experiments were conducted, using samples A, B, C, D, E, and F at a product dosage of 40 grams per run in a Bauknecht GSF 3162 dishwasher, with an intake of 5 litres water. The mainwash pH values for compositions A and B were adjusted to 7.5; the mainwash pH values for compositions C and D were adjusted to 8.5. Two - experiments (each) were conducted, using compositions E and F; the mainwash pH values were adjusted to 9.0 and 10.5, respectively. Silver-plated spoons, knives, and forks (supplied by Oneida Silversmiths, USA) , were used as monitors for all experiments. The monitors were washed in a commercially available hand dishwashing liquid and rinsed with deionized water and acetone before use. At the end of the dishwashing machine program, the monitors were visually evaluated for the presence of colours and/or loss of shine, according to the reference scale described in Example 1. Each sample containing purine (B, D and F) was observed to reduce the level of tarnishing relative to the samples which did not contain purine (A, C and E) . While tarnishing inhibition due to purine occurs through the pH range described above, this effect was observed to be greatest at pH values of 7.5, 8.5 and 9.0. Furthermore, the presence of purine reduced tarnishing regardless of the type of bleaching agent (chlorine or oxygen) present in the samples. EXAMPLE 5 The required association of the pK a value of purine class compounds usable in the invention and the pH of the composition in which they are incorporated to provide effective silver anti-tarnishing was demonstrated and the results are reported below. The pK. of each compound was determined by preparing a 0.001M solution of inhibitor in deionized water. The pH of this solution was adjusted to 3.0 with H 2 S0 4 . The solution was then titrated with IN NaOH to pH 11.0. A plot of mis. NaOH vs. pH for each sample was prepared. The pK a of the compound is that point where the maximum change in pH as a function of mis. NaOH is observed. A comparison of anti-tarnishing performance of purine class compounds within the scope of the invention and azole compounds outside the invention was conducted. The selected compounds were incorporated in an amount of 1 wt.% in a machine dishwashing composition containing 4 wt.% epsilon- phthalimido peroxyhexanoic acid as the bleaching agent. The pH values of the compositions were adjusted by the addition of a 50% solution of sodium hydroxide or concentrated sulfuric acid, as necessary. Silver plates were then held in each of the compositions for 25 minutes, removed, rinsed with deionized water and evaluated for silver tarnishing, and ranked as described in Example 1. A tarnish score of 3 or less was considered effective as a silver anti-tarnishing compound. The observations were tabulated as presented below: 36 Table 8 It was observed that the claimed compounds must exhibit a pK a of more than 1 unit less than the pH of an aqueous solution of the composition in which they are incorporated. In particular, adenine (pK a = 7.2) and guanine (pK :ι = 6.7) prevented silver tarnishing at pH 8.6, but did not prevent tarnishing at pH 6.9 and 7.0, respectively. Azole compounds outside the scope of the invention were tested and the following silver anti-tarnishing results were observed. Table 9 It was observed that compounds having pK a values greater than the pH values of the compositions in which they were incorporated did not exhibit anti-tarnishing effects. EXAMPLE 6 To demonstrate the ineffectiveness of known copper anti- tarnishing compounds on silver plates, machine dishwashing compositions having a pH of both 8.5 and 11 were prepared containing 4% by weight epsilon-phthalimido peroxyhexanoic acid as the bleaching agent and 1% by weight of three copper anti-tarnishing compounds listed below. Samples of the compositions were adjusted to both a pH of 8.5 and 11 by the addition of sodium hydroxide. Copper plates and silver plates were held in each of the compositions for 25 minutes, removed, rinsed with deionized water and evaluated for tarnishing. The following results were observed. Table 10 'Described as effective copper anti-tarnish compounds in US 2,618,608 (Schaeffer) It was observed that anti-tarnishing compounds which prevented tarnishing on copper plates had no effect on preventing silver tarnishing. The oxidation behaviour of copper versus silver is very different and compounds which affect one type of metal may be drastically different from those compounds which affect the other. The compounds exhibiting copper anti-tarnishing effects do not possess a structure within the scope of the anti-tarnishing compounds which are useful in the invention. EXAMPLE 7 Three machine dishwashing compositions were prepared as described in Example 1, except that a chlorine bleaching agent and cyanuric acid as anti-tarnish agent were combined as follows: Table 11 1 CDB, supplied by Olin Corporation of Stamford, Connecticut, USA 2 An 8.29% active Cl 2 solution, supplied by Jones Chemicals of Caledonia, New York, USA 3 Supplied by Aldrich Chemical Co. , of Milwaukee, Wisconsin, USA Tarnish monitoring experiments as described in Example 1 were conducted with compositions A to C using two silver- plated spoons as monitors in each of the experiments. The mainwash pH in this series of experiments was typically about 8.7. The influence of the compositions A, B and C on silver tarnishing are shown in the following Table 12. The anti- tarnish scale ranks from 0 to 6, according to the levels of tarnishing obtain with the compositions a to g of Example 1. TABLE 12 It was observed that spoons washed with composition C, containing 1% cyanuric acid, remained unchanged during the dishwashing process. In contrast, composition B, in which hypochlorite was incorporated without cyanuric acid, produced quite heavy tarnishing. Composition A containing sodium dichloroisocyanurate but no anti-tarnishing agent exhibited little silver tarnishing. It is believed that isocyanurate is formed during a wash in which sodium dichloroisocyanurate is used. Isocyanurate can be formed upon dissociation of dicholoroisocyanurate, producing hypochlorite as the active bleaching species. This is also believed to explain why chloroisocyanurate bleach systems generally do not cause many problems in terms of silver tarnishing. EXAMPLE 8 Six machine dishwashing compositions were prepared as described in Example 1, except that the bleaching agent and anti-tarnish agent were varied as follows: Table 13 A 32% peracetic acid solution supplied by Aldrich Corp. of Milwaukee, Wisconsin, USA Supplied by Hoechst AG of Germany A 8.2% active Cl 2 solution supplied by Jones Chemicals of Caledonia, New York, USA Supplied by Aldrich Corp. Tarnish monitoring experiments were conducted using compositions A, B, C, D E and F at a product dosage of 40 grams per run in a Bauknecht GSF 3162 dishwasher, with an intake of 5 litres water. The mainwash pH values for compositions A and B were adjusted to 7.5; the mainwash pH values for compositions C and D were adjusted to 8.5. Two experiments (each) were conducted using compositions E and F; the mainwash pH values were adjusted to 9.0 and 10.5, respectively. Silver-plated spoons, knives, and forks (supplied by Oneida Silversmiths, USA) , were used as monitors for all experiments. The monitors were washed in a 43 commercially available dishwashing liquid and rinsed with deionized water and acetone before use. At the end of the dishwashing machine program, the monitors were visually evaluated for the presence of colours and/or loss of shine, according to the reference scale described in Example 1. In each case, the presence of cyanuric acid reduced the level of tarnishing relative to the case where no anti- tarnish agent was present. While tarnishing inhibition due to cyanuric acid occurred throughout the pH range described above, this effect was greater at higher alkalinity. At pH 7.5, the introduction of cyanuric acid led to a reduction of 1.5 units of tarnishing; at pH 10.5 the reduction in tarnishing was 3.0 units. Furthermore, cyanuric acid reduced tarnishing irrespective of the type of bleaching agent present in the composition; inhibition was noted for both chlorine and oxygen bleaches. EXAMPLE 9 Five machine dishwashing compositions were prepared as described in Example 1, except that epsilon- phthali idoperoxyhexanoic acid was used as a peroxygen bleaching agent and cyanuric acid was used at five different amounts as follows: TABLE 14 Supplied by Hoechst AG, Germany Supplied by Aldrich Co. As described in Example 1, tarnish monitoring tests were conducted with compositions A to E using two silver-plated spoons as monitors in each of the experiments. The mainwash pH in this series of experiments was typically between 8.8 and 8.5. The effects of compositions A to E on silver tarnishing are shown in Table 15 below. The anti-tarnish scale ranks from 0 to 6, according to the levels of tarnishing obtained with the compositions a to g of Example 1. TABLE 15 It can be seen from this table that the level of cyanuric acid which gives optimum anti-tarnish performance is about 1%. The monitors had a slightly yellow appearance after using cyanuric acid at a 2.5% level. EXAMPLE 10 The following machine dishwashing compositions were prepared according to Example 1, except that 4% by weight of epsilon-phthalimidoperoxyhexanoic acid was incorporated as the bleaching agent and various azole-compound anti- tarnishing agents (both within and outside the scope of the definition of the 1,3-N azole compounds characteristic of the present invention) were incorporated in an amount of 1% by weight, as follows: TABLE 16 1 Compounds supplied by Aldrich Chemical Co. of Milwaukee, Wisconsin, USA Tarnish monitoring experiments were conducted as described in Example 1, using two silver-plated spoons as monitors in each of the experiments. The main wash pH in the experiments was between 8.8 and 8.5. The control composition (Sample A) produced heavy tarnishing on the spoons. Samples E, F and K (within the scope of the invention) exhibited only a slightly tarnished appearance on the washed spoons. Compositions G and H also exhibited only a slightly tarnished appearance on the washed spoons. In contrast, compositions B, C, D and J (outside the scope of the invention) exhibited heavy tarnishing on the washed spoons. EXAMPLE 11 Six machine dishwashing compositions were prepared as described in Example 1, except that the type of bleaching agent and the amount of 1,2,4-triazole as anti-tarnishing agent were varied as follows: TABLE 17 A 32% peracetic acid solution, supplied by Aldrich Corporation of Milwaukee, Wisconsin, USA was used. Supplied by Hoechst AG, Germany. A 8.2% active Cl 2 solution was used, supplied by Jones Chemicals of Caledonia, New York, USA. Tarnish monitoring experiments were conducted using compositions A, B, C, D, E and F at a product dosage of 40 grams per run in a Bauknecht GSF 3162 dishwasher, with an intake of 5 litres water. The mainwash pH values for compositions A and B were adjusted to 7.5; the mainwash pH values for compositions C and D were adjusted to 8.5. Two experiments (each) were conducted using compositions E and F; the mainwash pH values were adjusted to 9.0 and 10.5. respectively. Silver-plated spoons, knives and forks (supplied by Oneida Silversmiths, USA) , were used as monitors for all experiments. The monitors were washed in a commercially available dishwashing liquid and rinsed with deionized water and acetone before use. At the end of the dishwashing machine program, the monitors were visually evaluated for the presence of colours and/or loss of shine, according to the reference scale described in Example 1. In each case, the presence of 1,2,4-triazole reduced the level of tarnishing relative to the case where no anti- tarnishing agent was present. While tarnishing, inhibition due to 1,2,4-triazole occurred throughout the pH range described above, this effect was greater at higher alkalinity. At pH 7.5, the introduction of 1,2,4-triazole led to a reduction of 1.5 units of tarnishing; at pH 10.5 the reduction in tarnishing was 3.0 units. Furthermore, 1,2,4-triazole reduced tarnishing irrespective of the type of bleaching agent present in the composition; inhibition was noted for both chlorine and oxygen bleaches. EXAMPLE 12 The following machine dishwashing compositions were prepared as described in Example 1, except that epsilon- ' phthalimido peroxyhexanoic acid was included as the bleaching agent at a level of 4% by weight and various levels of 1, 2,4-triazole as anti-tarnish agent were incorporated as follows: TABLE 18 As described in Example 1, anti-tarnish monitoring tests were conducted using two silver-plated spoons as monitors. The main wash pH in these experiments was between 8.8 and 8.5. The effect of samples A to E on silver tarnishing was observed and is shown in Table 19 below: TABLE 19 It was thus observed that at amounts of 0.05 % by weight, the anti-tarnish agent 1,2,4-triazole effectively reduced silver tarnishing. Optimum performance of the agent was achieved at levels of about 1 % by weight. EXAMPLE 13 The required association of the pK a value of 1,3-N azole compounds usable in the invention and the pH of the composition in which they are incorporated to provide effective silver anti-tarnishing was demonstrated and the results are reported below. A comparison of anti-tarnishing performance of 1,3-N azole compounds within the scope of the invention and azole compounds outside the invention was conducted. The selected compounds were incorporated in an amount of 1 wt.% in a machine dishwashing composition containing 4 wt.% epsilon- phthalimido peroxyhexanoic acid as the bleaching agent. The pH values of the compositions were adjusted by the addition of a 50% solution of sodium hydroxide or concentrated sulfuric acid, as necessary. The pK a of each compound was determined by preparing a O.OOIM solution of inhibitor in deionized water. The pH of this solution was adjusted to 3.0 with H 2 S0 4 . The solution was then titrated with IN NaOH to pH 11.0. A plot of mis. NaOH vs. pH for each sample was prepared. The pK. of the compound is that point where the maximum change in pH as a function of mis. NaOH is observed. Silver plates were then held in each of the compositions for 25 minutes, removed, rinsed with deionized water and evaluated for silver tarnishing, and ranked as described in Example 1. A tarnish score of 3 or less was considered effective as a silver anti-tarnishing compound. The observations were tabulated as presented below: Table 20 It was observed that tetrazole was not effective as a silver anti-tarnishing at a pH of 8.5 but was effective at a pH of 9.5 and greater. Tetrazole with a pK a of 8.4 is effective only in compositions having a pH of greater than 9.4. 5-aminotetrazole having a pK a of 8.1 was not effective at a pH of 8.8. The other compounds exhibited effective anti-tarnishing effects because their pK a values were more than 1 unit less than the pH of an aqueous solution of the composition in which they were incorporated. Azole compounds outside the scope of the invention were tested and the following silver anti-tarnishing results were observed. Table 21 It was observed that compounds having pK a values greater than the pH values of the compositions in which they were incorporated did not exhibit anti-tarnishing effects. 2- Phenylimidazole did not prevent silver tarnishing at a pH 11.0, possibly because of hindrance from the phenyl group attached to the imidazole ring. EXAMPLE 14 To demonstrate the ineffectiveness of known copper anti- tarnishing compounds on silver plates, machine dishwashing compositions having a pH of both 8.5 and 11 were prepared containing 4% by weight epsilon-phthalimido peroxyhexanoic acid as the bleaching agent and 1% by weight of three copper anti-tarnishing compounds listed below. Samples of the compositions were adjusted to both a pH of 8.5 and 11 by the addition of sodium hydroxide. Copper plates and silver plates were held in each of the compositions for 25 minutes, removed, rinsed with deionized water and evaluated for tarnishing. The following results were observed. Table 22 1 Described as effective copper anti-tarnish compounds in US 2,618,608 (Schaeffer) It was observed that anti-tarnishing compounds which prevented tarnishing on copper plates had no effect on preventing silver tarnishing. The oxidation behavior of copper versus silver is very different and compounds which affect one type of metal may be drastically different from those compounds which affect the other. The compounds exhibiting copper anti-tarnishing effects do not possess a structure within the scope of the anti-tarnishing compounds which are useful in the invention.

Claims: CLAIMS 1. A bleaching detergent composition comprising: (a) from 1 to 20% by weight of a bleaching agent selected from a peroxygen or peroxygen-yielding compound, a hypohalite or hypohalite-yielding compound, or a salt thereof, or mixtures thereof; (b) from 0.05 to 10% by weight of an anti-tarnishing agent selected from: (i) a purine class compound of the following formula (I): wherein X 1 is nitrogen or C-R 3 , Y 1 is nitrogen or C-R 4 , and R 1 , R 2 , R 3 and R 4 are each independently hydrogen, hydroxy, alkoxy, amine, straight or branched chain alkyl having 1 to 20 carbon atoms, amido, amidoalkyl, alkylthio, alkenyl or hydroxyalkyl; (ii) cyanuric acid or isocyanuric acid or a salt thereof; (iii) a 1,3-N azole compound of the following formula (ii) ; wherein X 2 is C-R 7 or nitrogen provided Y 2 is also nitrogen, Y 2 is nitrogen or C-R 6 , and R 5 , R 6 and R 7 are each independently hydrogen, amine, amido, straight or branched chain alkyl having from 1 to 20 carbon atoms, an amino- or carboxylic-containing chain, alkoxy, alkylthio, hydroxy, hydroxyalkyl, alkenyl, or R 5 and R 6 taken together form an unsubstituted or substituted aryl group; or a salt thereof; (iv) a mixture of any of the above compounds (i) , (ii) or (iii) ; (c) optionally from 1 to 75% by weight of a detergency builder; and (d) optionally, from 0.01 to 40% by weight of a surfactant; wherein the composition exhibits a pH value in the range from 7 to 13, and wherein the anti-tarnishing agent has a pK a value below the pH value of an aqueous solution of the composition. 2. A composition according to claim 1, wherein the anti- tarnishing agent has a pK a value at least 1 unit below the pH value of an aqueous solution of the composition. 3. A composition according to claim 2, wherein the anti- tarnishing agent has a pK a value from 2 to 6 units below the pH value of an aqueous solution of the composition. 4. A* composition according to any one of claims 1 to 3 , wherein the anti-tarnishing agent comprises a compound of formula (I) wherein X 1 is nitrogen and Y 1 is C-R 4 and R 1 , R 2 , R 3 and R 4 are each independently hydrogen, hydroxy, alkoxy, oxygen, alkylthio, amine, amido or lower alkyl having from 1 to 6 carbon atoms. 5. A composition according to claim 4, wherein the compound of formula (I) is selected from purine, adenine, guanine, 6-mercaptopurine, xanthine, hypoxanthine, -uric acid, and allopurinol. 6. A composition according to any one of claims 1 to 3 , wherein the anti-tarnishing agent comprises a 1,3-N azole compound of formula (II) wherein X 2 is C-R 7 , X 2 is nitrogen provided Y is nitrogen, Y 2 is C-R 6 and R s and R 6 taken together form an aryl or a substituted aryl group, or a 1,3-N azole compound of formula (II) wherein Y 2 is C-R 6 and R 5 and R 6 are each independently hydrogen, amine, amido, straight or branched chain alkyl having from 1 to 6 carbon atoms, alkoxy, alkylthio, hydroxy, alkenyl or an amino- or carboxylic-containing moiety. 7. A composition according to claim 6, wherein the compound of formula (II) is selected from imidazole, benzimidazole, tetrazole, 5-aminotetrazole, 1,2,4-triazole, 3-amino-l,2,4-triazole and histidine. 8. A composition according to any preceding claim, which has a pH value in aqueous solution of from 7 to 11. 9. A composition according to any preceding claim, wherein the bleaching agent comprises a peroxy compound selected from the group consisting of: organic mono- and di-peroxyacids; organic diacylperoxides; and inorganic peroxy-containing salts. 10. A composition according to any preceding claim, wherein the bleaching agent comprises a peroxyacid bleach precursor. 11. A composition according to any preceding claim, wherein the bleaching agent comprises a halogen-containing compound selected from the group consisting of: hypohalite salts; active halogen-containing compounds which yield hypohalite anions in aqueous alkaline solution; and halogenated isocyanuric acids. 12. A composition according to any preceding claim, which further comprises one or more additional ingredients selected from the group consisting of: silicates; filler materials; thickeners; stabilizers and/or co-structurants; defoaming agents; and enzymes. 13. A method of washing a silver or silver-plated article whilst preventing or inhibiting tarnishing thereof during or as a result of the washing process, the method comprising washing the said article with a bleaching detergent composition according to any one of claims 1 to 12. 14. Use of a compound selected from any of (i) to (iv) defined in claim 1 as a silver anti-tarnishing agent in a peroxygent- and/or hypohalite-based bleaching detergent composition having a pH value in the range of from 7 to 13.

Inventors: ANGEVAARE PETRUS ADRIANUS J M, GARY RICHARD GERALD, ANGEVAARE, PETRUS, ADRIANUS, J., M., GARY, RICHARD, GERALD
Applicants: UNILEVER NV, UNILEVER PLC, UNILEVER N.V.
Application year: 1994
Main classification: C11D3
IPC codes: C11D3